Need legal corporate help!

Sincerely need some help ! Ive got an exciting offer but they want me join within 30 days I've communicated with my manager he told if he can get my replacement he can release me early. I filed resignation with comments of requesting release under 30 days 3 days later my manger denied for early release. How can I get early release I am ready for Np buyout but management says np buyout will be approved only if manager approves. I tried to push the matter but HR warned if you push it more you may be blacklisted. Do you think Legal help would work here?